A telecommunication network power system usually includes an ac-dc stage converting the power from the ac utility line to a 48V dc distribution bus and a dc/dc stage converting the 48V dc distribution bus to a plurality of voltage levels for all types of telecommunication loads. A conventional ac-dc stage may comprise a variety of EMI filters, a bridge rectifier formed by four diodes, a power factor correction circuit and an isolated dc/dc power converter. The dc/dc stage may comprise a plurality of isolated dc/dc converters. Isolated dc/dc converters can be implemented by using different power topologies, such as LLC resonant converters, flyback converters, forward converters, half bridge converters, full bridge converters and the like.
In a telecommunication network power system, isolated dc/dc converters may generate common mode noise. More particularly, an isolated dc/dc converter may comprise at least one primary side switch to chop an input dc voltage so as to generate an ac voltage across the primary side of a transformer. In order to achieve a compact solution, the isolated dc/dc converter may operate at a high switching frequency such as 1 MHz. Such a high switching frequency may generate a high and fast voltage swing across the primary side. Furthermore, there may be a plurality of parasitic capacitors coupled between the primary side and the secondary side of the transformer. The high frequency voltage swing and the parasitic capacitors lead to common mode noise in an isolated dc/dc converter because the parasitic capacitors of the transformer provide a low impedance conductive path for common mode current derived from the high frequency voltage swing.
In order to control the electromagnetic interference (EMI) pollution from common mode noise, a variety of international standards have been introduced. For example, EMI standard EN55022 Class B is applicable to isolated dc/dc converters. In accordance with a conventional technique, a common mode choke may be employed to attenuate common mode noise. The common mode choke may be placed between an input dc voltage source and the primary side switching network of an isolated dc/dc converter. The common mode chock can pass the dc current from the input dc source while blocking the common mode current generated from the primary side switching network. However, an effective common mode choke may be of a large inductance value, which may require a big and expensive coil. Such a big and expensive coil may increase the size of the isolated dc/dc converter, introduce extra cost and degrade the efficiency of the isolated dc/dc converter as well.